PEER-REVIEW ARTICLE PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7367 Effect of Metal Oxides on Reaction Route and Product Distribution of Catalytic Cellulose Hydrogenolysis Guifang Fan,a,b,* De Chen,c Shizhong Li,a,b Mingde Yang,a and Yulong Wu a The effects of CeO2, ZrO2, Nb2O5, and ZnO catalysts supported on carbon nanotubes (CNT) relative to cellulose hydrothermal hydrogenolysis in the presence of Ni/CNT and pressured H2 was studied in this work. The catalysts were characterized by inductively coupled plasma – optical emission spectrometry, X-ray diffraction, X-ray photoelectron spectrometry, transmission electron microscopy, NH3 temperature programmed desorption (TPD), and CO2-TPD. Glucose and its isomers were detected by mass spectrometry. The results showed that redox active CeO2/CNT with strong Lewis acid and strong Lewis base sites was active in C-C bong cracking, isomerization, dehydrogenation, and hydrodeoxygenation reaction, yielding 36.3% ethylene glycol and 17.2% 1,2-propylene glycol. The ZnO/CNT with Bronsted base accelerated isomerization, retro-aldol condensation, and dehydrogenation, yielding 20.7% 1,2-propylene glycol, 17.8% ethylene glycol, and 12.7% tetrahydrofuran dimethanol. The Nb2O5/CNT and ZrO2/CNT were inert to C-C bond cracking, whereas H+ in hot compressed water and the Bronsted acid in Nb2O5/CNT accelerated dehydration, yielding more sorbitol and sorbitans. The results provide reference for catalyst selection and product regulation in cellulose hydrogenolysis. DOI: 10.15376/biores.18.4.7367-7390 Keywords: Cellulose hydrogenolysis; CeO2; ZnO; ZrO2; Nb2O5 Contact information: a: Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China; b: Beijing Engineering Research Center of Biofuels, Tsinghua University, Beijing, 100084, China; c: Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU) N-7491 Trondheim, Norway; *Corresponding author: fanguifang@tsinghua.edu.cn INTRODUCTION Biomass conversion, if carried out appropriately, offers the attractive feature of carbon neutrality. Catalytic degradation of cellulose to chemicals has attracted much attention in recent years. Hydrogenation can transform generated glucose to more stable compounds such as sorbitol, prevent the formation of humin (Maruani et al. 2018), and then increase the yields of products. Cellulose could be selectively converted to sorbitol, ethylene glycol (EG), and 1,2-propylene glycol (1,2-PG) by catalytic hydrogenation reactions (Sun et al. 2016; Lazaridis et al. 2017; Li et al. 2017; Gu et al. 2019; Li et al. 2019; Zan et al. 2019; Zhang et al. 2019; Zheng et al. 2017). By far, the highest yields of sorbitol, EG, and 1,2-PG are reported as 91% (Shrotri et al. 2018), 77.5% (Li et al.2018), and 39% (Xiao et al. 2013), respectively. Sorbitol has been identified as one of the twelve most important building blocks derived from biomass resources, EG and 1,2-PG are raw materials for polymer industry. Cellulose catalytic hydrogenation into chemicals is an attractive alternative in its valorisation. The catalytic hydrogenation of cellulose mainly includes the following steps: (1) cellulose hydrolyzed to glucose in the presence of H+, (2) glucose hydrogenated to sorbitol PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7368 with the catalysis of Ni, or noble metal atom, (3) sorbitol dehydrogenated back to hexose, or dehydration to sorbitan, (4) glucose isomerized to fructose in the presence of base or Lewis acid (Delidovich and Palkovits 2016; Nguyen et al. 2016), (5) glucose and fructose experienced Retro-Aldol Condensation (RAC), C-C breaking took place, formed lower aldose and ketose such as erythrose, glycolaldehyde, glyceraldehyde, 1,3- dihydrocyacetone, etc. (6) The products of RAC have been hydrogenated and dehydrated to EG, 1,2-PG, etc. Isomerization and RAC are accelerated by base catalyst; however, homogeneous alkali catalysis will neutralize with H+ and hinder cellulose hydrolysis (Li et al. 2015). Metal oxides possess Lewis acid and basic sites, which catalyze isomerization and RAC reaction with no discount of cellulose conversion. The acid-base property of sparingly soluble oxides in contact with aqueous solutions are chiefly determined by the isoelectric point of solid surface (IEPS). For some metal oxides, the IEPS values are rather constant. The IEPS values of CeO2, ZnO, ZrO2, and Nb2O5 were reported as being 8.1, 9.2, 7.8, and 4.1 (Kosmulski 1997). These findings mean that suspension of ZnO and Nb2O5 are Brønsted base and acid, respectively. CeO2 is known as an oxygen storage substance (Montini et al. 2016). ZrO2 is a compound rich in Lewis acid and base sites (Huang et al. 2019). Carbonaceous materials are hydrothermal stable even at elevated temperatures (Pham et al. 2015). Carbon nanotubes and surface functionalized biochar were found to work well as catalyst support on hydrothermal conversion of biomass (Chen et al. 2017; Liu and Liu 2020). The effects of typical metal oxides catalysts such as CeO2/CNT, ZnO/CNT, ZrO2/CNT, and Nb2O5/CNT on the reaction route and product distribution of cellulose catalytic hydrogenation were studied in this paper. EXPERIMENTAL Materials Microcrystalline cellulose, Ce(NO3)3.6H2O, ZrO(NO3)2.6H2O, NbCl5, Ni(NO3)2.6H2O, Zn(NO3)2.6H2O, citric acid, EG, 1,2-PG, tetrahydrofuran dimethanol (THFDM), sorbitol, glucose, mannose, fructose, and 1,4-sorbitan were purchased from Sigma-Aldrich. They were all analytical reagents. Carbon nanotubes (CNT) were from Chengdu Organic Chemicals Co. Ltd, China. Preparation of Catalyst To remove mineral impurities, CNT was pretreated with 65% nitric acid as previously described (Van der Wijst et al. 2015), briefly, boiled for 30 min, then washed with deionization water until the filtrate was neutral. A complex solution was composed by mixing the metal salt (Ce(NO3)3.6H2O, ZrO(NO3)2.6H2O, NbCl5, Ni(NO3)2.6H2O, or Zn(NO3)2 •6H2O), citric acid (CA), EG and deionized water in an ultrasonic bath for 10 min. The molar ratios between the three components Metal:CA:EG used were 7:8:8. The complex solution was introduced to the CNTs in isometric impregnation, with the ratio of 4.32 mmol metal to 1 g CNT. Metal salts/CNT precursor dried in room temperature for 12 h and at 110 °C for 12 h, then calcinated for 10 min at 400 °C in air flow to form metal oxide (MOx)/CNT. The heating and cooling were done in pure N2 flow with a heating rate of 10 °C/min. For NiO/CNT, reduction was carried out in H2 gas for 5 h at 400 °C to form Ni/CNT. PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7369 Characterization of Catalyst The contents of metal in the catalyst were determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) (Thermo IRIS intrepid Ⅱ). 0.010 g of catalyst was dissolved in acid solution (40% HF for ZrO2/CNT and Nb2O5/CNT, 6 mol/L HNO3 for CeO2/CNT, diluted HNO3 for Ni/CNT and ZnO/CNT), digested for 60 min (90 °C for CeO2/CNT and ZrO2/CNT, 25 °C for other catalysts), and diluted with water to 25 mL. 1 mL of supernatant was taken, diluted to the second 25 mL, and the solution was collected for the detection. X-ray photoelectron spectroscopy (XPS) analyses were carried out with a PHI Quantera SXM X-ray photoelectron spectrometer using a monochromatic Al-Ka X-ray source. The phase structure of the catalyst was determined by powder X-ray diffraction (XRD) spectroscopy (Bruker D8 Advance) using a Cu-Ka radiation (k = 0.154 nm), the data ranging from 5 to 80 were collected at a step size of 0.02, and the particle size was investigated using the Scherrer Equation. The surface morphology and structure of the catalyst was collected by transmission electron microscope (TEM) (HITACHI-HT 7700). Surface acid (base) properties of the catalysts were probed by NH3 (CO2)- Temperature Programmed Desorption (TPD), CO2-TPD was performed on a Cat-Lab instrument (BEL, Japan) equipped with a well-calibrated quadrupole mass spectrometer (MS) (Inprocess Instruments, GAM 200) as the detector. The CO2 desorption profiles were obtained by recording the signal for molecular CO2 (m/z=44). NH3-TPD was performed on TPD-TPR instrument (Xianquan, China TP-5076) equipped with a TCD detector. The sample of 40 mg (50 mg) was purged with dry Ar (He) at 500 °C for 1 h, followed by reducing the reactor temperature to 100 °C (50 °C) and switching to a flow of 20% CO2/Ar (10% NH3/He) for 1 h to execute CO2 (NH3) adsorption. After purging for 1.5 h at 100 °C (several minutes at 50 °C) with flowing Ar (He) until the signal was constant, the sample was heated to 500 °C at a rate of 10 °C min-1, then kept for 30 min (90 min) to allow for desorption of adsorbed CO2 (NH3). MS intensity ratio of CO2 to Ar and the volume of Ar passed were used to quantify the CO2 desorption amounts. For NH3, the calibration was performed by injecting pulses of 10% NH3/He. The amounts of the acid and basic sites were analyzed based on mathematical deconvolution of the TPD profiles. Procedure for Cellulose Conversion The catalytic reactions were tested in a stirring batch reactor (Parr Instruments Company). Typically, a 300 mL batch reactor was used with deionized water (100 mL), microcrystalline cellulose (1.00 g), catalyst (Ni/CNT+MOx/CNT with the amount of Ni and MOx to be 0.81 mmol) and an initial H2 pressure of 60 bar. The reactor was closed and heated to T=245°C with a heating rate of 2 °C /min, which generated a pressure of 120 bar, and the system was kept at T=245 °C for 270 min. When the temperature was ramped to 180, 210, and 240 °C, three samples were taken marked as 180C, 210C, and 240C. Alternatively, when the reaction went on for 15, 30, 45, 90, 150, 210, and 270 min at 245 °C, samples taken were marked as 15M, 30M, 45M, 90M,150M, 210M, and 270M. After the reaction, the catalyst was retrieved from the reaction mixture using a quantitative ashless filter paper. The conversion of cellulose was calculated according to equation (1). The yields of product were calculated according to equation (2). The catalytic reactions were duplicated, the relative standard deviations of product yield were below 5% (yield>0.05). PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7370 𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 = 𝑊𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒 𝑎𝑑𝑑𝑒𝑑+𝑊𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 𝑎𝑑𝑑𝑒𝑑−𝑊𝑟𝑒𝑠𝑖𝑑𝑢𝑒 𝑊𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒 𝑎𝑑𝑑𝑒𝑑 × 100% (1) Product yield: 𝑌 = 𝑚𝑝𝑟𝑜𝑑𝑢𝑐𝑡×(𝑚𝑎𝑠𝑠 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛 𝑖𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡) 𝑚𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒×(𝑚𝑎𝑠𝑠 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛 𝑖𝑛 𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒) (2) where Wcellulose added and Wcatalyst added represent the weight (g) of cellulose and catalyst added, Wresidue represents the weight (g) of residue retrieved, containing used catalyst, mproduct represents the mass of product (g), for which the products were EG,1,2-PG, sorbitol, THFDM, and 1,4-sorbitan, mcellulose represents the mass of cellulose added (g). Analysis of Products HPLC (Agilent Technologies 1260 Infinity)-RID (differential refractive index detector) with a Hi-Plex Ca (Duo) 300 x 6.5 mm column was used to quantitatively detect the product, the concentration of product was obtained by one-point external standard method with the content of standard solution below 0.4 mg/mL. The relative standard deviation of the analytical method was below 1% (yield >0.05). To monitor glucose and its isomer, SIM mode evaluations (m/z=203) of all samples were conducted with a SHIMADZU LCMS 2010 EV. RESULTS AND DISCUSSION Characterization of the Catalyst The contents of metal in catalysts were detected by ICP-OES. As shown in Table S-1, the contents were close to those calculated by the amount of reactant. They were from 2.29 to 3.05 mmol/g. The valences of metal in catalysts were identified by XPS spectrum (Fig S-1 to S-4). The binding energy of Ce3d indicated that CeO2 and Ce2O3 were coexistent, and other metal oxides were identified to be ZnO, Nb2O5, and ZrO2. The XRD patterns of catalyst are shown in Fig. 1. 10 20 30 40 50 60 70 80 90            ZnO/CNT Ni/CNT ZrO2/CNT Nb2O5/CNT In te n s it y ( a rb . u n it s ) 2θ(degrees) CeO2/CNT  C-Graphite  CeO2  Ni  ZnO  r    Fig. 1. XRD profile of the catalysts PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7371 Metal oxide diffraction peaks were observed in profile of Ni/CNT (Ni, cubic, PDF 00-004-0850), ZnO/CNT (ZnO, Hexagonal, PDF 01-075-0576), and CeO2/CNT (CeO2, cubic, PDF 01-075-0076). The particle size calculated by the Scherrer Equation were 12.4, 17.5, and 3.3 nm, respectively. No diffraction peaks for metal oxides were observed in profile of ZrO2/CNT and Nb2O5/CNT, indicating that ZrO2 and Nb2O5 were amorphous. The morphology of the catalysts was collected by TEM (Fig. 2). TEM images showed that the particle sizes of Ni, ZnO, CeO2, Nb2O5, and ZrO2 were 14.5, 15.1, 5.7, 4.7, and 3.0 nm, respectively. (a) (b) (c) (d) (e) Fig. 2. TEM graph of the catalysts. (a). Ni/CNT; (b). ZnO/CNT; (c). CeO2/CNT; (d). ZrO2/CNT; (e). Nb2O5/CNT TPD was performed to probe Lewis acid and base sites of the catalysts. Table 1, Fig. S-5, and Fig.S-6 show the acid (base) concentration and strength of different catalysts. The most abundant base sites were found in CeO2/CNT, including weak, medium, and strong in strength. A few weak, medium, and strong basic sites were found in ZnO/CNT. PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7372 Many weak and a few strong basic sites were found in ZrO2/CNT. Only strong basic sites in Nb2O5/CNT were observed. A wide distribution of surface base sites from weak to strong were probed in all four catalysts, but the abundance in CeO2/CNT was far more than in the other three catalysts. Table 1. Concentration of Weak, Medium, and Strong Base and Acid Sites for Four Catalysts Catalyst Base sites (µmol CO2 g-1) Acid sites (µmol NH3 g-1) Weak Medium Strong total Weak medium strong total CeO2/CNT 8.55 9.28 12.64 30.47 5.41 5.23 13.39 24.03 ZrO2/CNT 4.35 - 2.62 6.97 2.95 0.60 0.50 4.05 Nb2O5/CNT - - 2.62 2.62 3.88 1.14 0.20 5.22 ZnO/CNT 0.36 0.51 1.27 2.14 0.86 - 1.20 2.06 Weak: Desorption temperatures below 200 °C. Medium: Desorption temperatures at 200–400 °C. Strong: Desorption temperatures above 400 °C. Product Distribution The cellulose conversion The cellulose conversion was calculated based on the relative change of weight before and after the reaction. They were 100% for all four types of catalysts when the reaction time was 270 min. When the reaction time was 30 min, cellulose conversions were 70.0%, 75.7%, 79.1%, and 91.2% for ZrO2/CNT, CeO2/CNT, Nb2O5/CNT, and ZnO/CNT. Glucose and its isomerization Isomerization have been demonstrated to take place in hot compressed water (Lu et al. 2012; Yan et al. 2021), Lewis acids such as metal salt (Nguyen et al. 2016), heterogeneous metal-substituted BEA zeolites (Gounder and Davis 2013; Bermejo-Deval et al. 2014), alkaline solution (Speck 1958), and various other media (Nagorski and Richard 2001; Saravanamurugan and Riisager 2014; Murzin et al. 2017). 1,2-hrdride transfer and 1,5-hydride transfer of glucose were observed in the Lewis Acid solution (Gounder and Davis 2013; Nguyen et al. 2016), to form fructose and sorbose, respectively. In light of their low concentrations and overlap with the corresponding values for of other substances in liquid chromatography, glucose and its isomers were detected by MS signals with m/z=203. The results are shown in Fig. 3 and Table S-2. There were three main isomers: glucose (Retention Time (Rt)=18.94), mannose (Rt=21.15), and fructose (Rt=22.48). No sorbose or galactose were detected. Mannose could be an inverse isomerization product of fructose. No mannose was detected with the catalyst CeO2/CNT. Two new peaks (Rt=25.10 and 26.45) were observed with the catalyst CeO2/CNT and Nb2O5/CNT, for which the Rt corresponded to tagatose and an unknown substance. Hexoses were increased in the former stage of the reaction and decreased later. Little hexose was detected at the end of reaction. The concentration of glucose varied greatly in reactions using different catalysts. The highest was in ZrO2/CNT (reaction time=30 min; yield<0.005), followed by Nb2O5/CNT and ZnO/CNT, and the lowest was in CeO2/CNT. The strength and amount of surface acid and base site could accelerate the consumption of glucose. The CeO2/CNT was found to possess a large amount of strong acid and base sites, leading to the lowest hexose concentration. Glucose was isomerized partially to fructose in ZnO/CNT and CeO2/CNT. PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7373 Fig. 3. LC-MS (m/z=203) of samples in different reaction time. (a)CeO2, Rt=19.21, 21.76,22.65, 25.10 and 26.45 min. (b) ZnO, Rt= 18.94, 21.15 and 22.48 min. (C)Nb2O5, Rt=18.87, 21.15， 22.41, 24.70 and 26.07 min. (d) ZrO2, Rt=18.91, 21.08 and 22.49 min Yield variation of main products in reaction time The yield variations of sorbitol, EG, 1,2-PG, THFDM, and 1,4-sorbitan in reaction time with different catalysts are shown in Fig. 4. The yields of sorbitol increased at the former stage of the reaction, but decreased as time went on for all four catalysts. The optimal reaction time was 90 min for sorbitol. The highest sorbitol yield was 31.1%, obtained for ZrO2/CNT at 90 min. Sorbitol yields were in the range of 16.8 to 53.4%, when carbon blacks, activated carbon, Al2O3, ZrO2, or TiO2 supported Pt catalyst were employed (Kobayashi et al. 2011). In systems where acid- functionalized carbonaceous materials or HZSM-5 served as support, and where Pt, Ru, or Ni served as the active component, the sorbitol yields were in the range of 39.4 to 70.0% (Manaenkov et al. 2019). A sorbitol yield of 91.0% was obtained with CuO/CeO2-ZrO2 (Manaenkov et al. 2019). Sorbitol could be decreased by dehydrogenation (Deutsch et al. 2012; Jia and Liu 2016), dehydration (Sun et al. 2013), hydrogenolysis (Sun et al. 2015), etc. The yields of EG increased at the former stage of the reaction, and they kept constant for ZrO2/CNT, ZnO/CNT, and Nb2O5/CNT after 90 min, but decreased for CeO2/CNT after 150 min. The highest EG yield was 36.3%, obtained for CeO2/CNT at 150 min. It is widely accepted that W-containing catalysts have the tendency of producing high (a) (b) (c) (d) PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7374 EG yields. The EG yields by using of many W-containing catalysts were reviewed; they were in the range of 8.4 to 77.5% (Manaenkov et al. 2019). 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 S o rb it o l y ie ld Reaction Time (min) CeO 2 Nb 2 O 5 ZnO ZrO 2 0 100 200 30050 150 2500 Ramping a) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 E th y le n e g ly c o l y ie ld Reaction Time (min) CeO 2 Nb 2 O 5 ZnO ZrO 2 0 100 200 3000 50 150 250 Ramping b) 0 50 100 150 200 250 300 0.00 0.04 0.08 0.12 0.16 0.20 P ro p y le n e g ly c o l y ie ld Reaction Time (min) CeO 2 Nb 2 O5 ZnO ZrO2 c) 0 50 100 150 200 250 300 0.00 0.03 0.06 0.09 0.12 0.15 0.18 T H F D M y ie ld Reaction Time (min) CeO 2 Nb 2 O 5 ZnO ZrO 2 d) 0 50 100 150 200 250 300 0.00 0.02 0.04 0.06 0.08 0.10 0.12 1 ,4 -S o rb it a n y ie ld Reaction Time (min) Nb 2 O 5 ZrO 2 e) Fig. 4. The yields of some products in different reaction time. Reaction conditions: P = 60 bars, H2 at room temperature (RT), T= RT to 245 °C with a heating rate of 2 °C /min, kept at 245 °C for 270 min. Microcrystalline cellulose (1.00 g), catalyst ((Ni/CNT+MOx/CNT with Ni and MOx to be 0.81 mmol) and distilled water (100 mL) in a 300 mL autoclave reactor. The yields of 1,2-PG increased at the former stage of the reaction, and kept constant for ZrO2/CNT and Nb2O5/CNT after 90 min, kept increased for ZnO/CNT after 90 min, and decreased for CeO2/CNT after 150 min. The highest 1,2-PG yield was 20.7%, obtained for ZnO/CNT at 270 min. High 1,2-PG yields were obtained in CuCr catalysts (Xiao et al. 2013) and Sn-containing catalysts (Manaenkov et al. 2019); the yields were in the range of 32.2 to 39.0%. (a) (b) (c) (d) (e) PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7375 The yields of THFDM kept increasing all the time for ZnO/CNT, Nb2O5/CNT, and ZrO2/CNT. The rate of increase for ZnO/CNT was higher than those for Nb2O5/CNT and ZrO2/CNT after 150 min. No THFDM was detected for CeO2/CNT. The highest yield was 13.6%, obtained for ZrO2/CNT at 270 min. Niobic acid and a ruthenium catalyst were used in glucose conversion, the selectivity to THFDM was 60% and conversion was 49% (Duan et al. 2017). 1,4-sorbitan was only observed for Nb2O5/CNT and ZrO2/CNT, the yields were 10.9% and 9.0%, respectively. Layered niobium molybdate (HNbMoO6) (Morita et al. 2014) and sulfuric acid (Yabushita et al. 2015) were used in the dehydration of aqueous- phase sorbitol, where the yields of 1,4-sorbitan were 33% and 58% respectively. The yields of 1,4-sorbitan kept increasing continually, except for a decline for Nb2O5/CNT after 210 min. The dehydration of sorbitol to 1,4-sorbitan was an acid-catalyzed reaction (Sun et al. 2013). Discussion The yields of EG, 1,2-PG, sorbitol, and sorbitans were mainly influenced by several types of reaction, as shown in Fig. 5. RAC of glucose produces EG. RAC of fructose produces 1,2-PG. Hydrogenation of glucose produces sorbitol. Dehydration of sorbitol produces 1,4-sorbitan, isosorbitan, and THFDM. Dehydration of fructose produces hydroxymethylfurfural (HMF) (Aida et al. 2007). The HMF is then hydrogenated to THFDM. ZnO/CNT resulted in the fastest cellulose hydrolysis rate. The isomerization of glucose to fructose (consumption of product) helped the hydrolysis reaction going forward. For CeO2/CNT, though the glucose consumption was fastest, glucose experienced C-C bond cracking, resulting in many small molecules, which occupied active H+ and active sites. These effects tempered the cellulose hydrolysis. No glucose or fructose was detected after 90 min for all four catalysts, which implied that cellulose hydrolysis had completed at 90 min. The yields of EG and 1,2-PG were increased for CeO2/CNT from 90 min to 150 min. This effect could have originated from RAC of pentose, or terminal C-C scission of glyceraldehyde or erythrose. Glucose could be oxidized to gluconic acid for the ability of CeOx to shuttle between Ce(III) and Ce(IV) state (Montini et al. 2016), then decarboxylation took place (Bohre et al. 2019), generating terminal C-C scission. Parts of EG and 1,2-PG were hydrodeoxygenated to ethanol and propanol after 150 min. For ZnO/CNT, sorbitol decreased, EG kept constant, and 1,2-PG increased after 90 min, which demonstrated that sorbitol was dehydrogenated at the 2/5-position, not the 1,6- position, which in accordance with the conclusion that a sorbitol dehydrogenation step proceeded by preferential activation of its C (5)–H bond (Jia and Liu 2016). For Nb2O5/CNT and ZrO2/CNT, EG and 1,2-PG were constant after 90 min, which implied that dehydrogenation of sorbitol was hard to achieve. The increasing rate of THFDM yield was lower after 90 min, which disclosed that the reaction rate of fructose to THFDM was faster than sorbitol to THFDM. Parts of 1,4-sorbitan had been converted to isosorbitan for Nb2O5/CNT after 210 min. PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7376 Fig. 5. Main reaction routes for cellulose hydrothermal hydrogenolysis. (Cel: Cellulose; Glu: Glucose; Fru: Fructose; THFDM: tetrahydrofuran dimethanol; Sor: Sorbitol; EG: Ethylene glycol; 1,2-PG: Propylene glycol. R1: hydrolysis; R2: isomerization; R3: retro-aldol condensation; R4: hydrogenation; R5: dehydrogenation; R6: dehydration). Less EG and 1,2-PG were produced for Nb2O5/CNT and ZrO2/CNT. 23.9% EG and 7.4% 1,2-PG were formed by Ni/CNT alone in the authors’ previous study (Van der Wijst et al. 2015). Nb2O5/CNT and ZrO2/CNT were found to be inert to C-C bond cracking. Liu’s studies showed that the crystalized WO3 was essential to C-C scission (Liu et al. 2012, 2022). Nb2O5/CNT and ZrO2/CNT were amorphous, which may cause inertness. However, in Gromov’s study, the main product of cellulose hydrogenolysis on hexagonal Nb2O5 and on monoclinic or tetragonal ZrO2 was also sorbitol (Gromov et al. 2021). It follows that the inertness of Nb2O5/CNT and ZrO2/CNT could be attributed to the surface acid/base property. CONCLUSIONS 1. The supported catalyst CeO2/carbon nanotube (CNT) was found to be rich in strong Lewis acid and base sites, and the crystallized CeO2 was redox active. In addition, it was active in C-C bond cracking (retro-aldol condensation (RAC), terminal C-C scission), isomerization, dehydrogenation, and hydrodeoxygenation reactions. The reaction yielded 36.3% ethylene glycol (EG) and 17.2% 1,2-propylene glycol (1,2-PG). 2. ZnO/CNT with Brønsted base accelerates isomerization, RAC, and dehydrogenation, yielding 20.7% 1,2-PG, 17.8% EG, and 12.7% THFDM. 3. Nb2O5/CNT and ZrO2/CNT were found to be inert to C-C bond cracking, H+ in hot compressed water and the Bronsted acid in Nb2O5/CNT accelerated dehydration, yielding more sorbitol and sorbitan. 4. The reaction network for cellulose hydrogenolysis is complex. The type of metal oxides influences glucose evolution and final product distribution greatly. Reaction time and catalysts should be cautiously selected to get a certain product. 1,2-PG EG R3 R3 Sor 1,4-sorbitan R6 Glu Fru THFDM Cel R1 R6 R2 R4 R4/R5 R6 PEER-REVIEWED ARTICLE bioresources.com Fan et al. 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DOI: 10.1021/acscatal.6b03469 Article submitted: July 28, 2023; Peer review completed: August 19, 2023; Revised version received and accepted: August 30, 2023; Published: September 8, 2023. DOI: 10.15376/biores.18.4.7367-7390 PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7381 APPENDIX Table S-1. Metal Content of Catalysts Catalysts Metal Contenta (mmol/g) Metal Contentb (mmol/g) Ratio (b/a） Ni/CNT 3.25 3.05 0.94 CeO2/CNT 2.48 2.29 0.92 ZrO2/CNT 2.82 2.37 0.84 Nb2O5/CNT 2.75 2.46 0.89 ZnO/CNT 3.20 2.92 0.91 a means metal content calculated by the amount of adding. b means metal content detected by ICP-OES. Table S-2. MS (m/z=203) Peak Area of Samples Catalysts Glucose Peak Area 180C 240C 15M 30M 45M CeO2/CNT - 404294 265376 133285 - Nb2O5/CNT 126647 2089765 1523861 1301997 1356869 ZrO2/CNT 50191 1014821 3446931 3699475 2526217 ZnO/CNT - 498434 1263747 881790 266261 Catalysts Fructose Peak Area 180C 240C 15M 30M 45M CeO2/CNT - 695247 395240 392575 272952 Nb2O5/CNT - 226605 495121 463051 488526 ZrO2/CNT - 563771 1379152 1310923 883998 ZnO/CNT - 476620 1197794 837857 345643 Catalysts Mannose Peak Area 180C 240C 15M 30M 45M CeO2/CNT - - - - - Nb2O5/CNT - 293616 212858 365254 379649 ZrO2/CNT - 376379 977200 915544 604114 ZnO/CNT - 522400 816764 608119 189402 PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7382 Fig. S-1. Binding energy of Ce3d of Ce (IV)O2+Ce2(Ⅲ)O3/CNT Binding Energy (eV) PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7383 Fig. S-2. Binding energy of Zn2p of ZnO/CNT Binding Energy (eV) PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7384 Fig. S-3. Binding energy of Nb3d of Nb2O5/CNT Binding Energy (eV) PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7385 Fig. S-4. Binding energy of Zr3d of ZrO2/CNT Binding Energy (eV) PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7386 500300 ZnO/CNT 3.0E-5 a.u. 100 isothermic Temperature (C) 100 300 Nb 2 O 5 /CNT 4.0E-5 a.u. 500 isothermic Temperature (C) 1.1E-4 a.u. ZrO 2 /CNT C O 2 D e s o rp ti o n ( a .u .) C O 2 D e s o rp ti o n ( a .u .) 2.5E-4 a.u. CeO2/CNT Fig. S-5. CO2-TPD of different catalysts 0 2 4 6 8 in te n s it y ( a .u .) Temperature (C) CeO2/CNT ZnO/CNT Nb2O5/CNT ZrO2/CNT Blank 0 500100 300 500 90 min Fig. S-6. NH3-TPD of different catalysts In te n s it y ( a .u .) PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7387 0 50 100 150 0.0 2.0x10 0 4.0x10 0 6.0x10 0 8.0x10 0 N H 3 -T P D S ig n a l o f C e O 2 /C N T (a .u .) Time (min) Peak Analysis BaseLine:Line Adj. R-Square=-- # of Data Points=% ([Book1]FitPeaks1,@WL,RegStats.C1.N) Degree of Freedom=% ([Book1]FitPeaks1,@WL,RegStats.C1.DOF)SS=-- Chi^2=-- Date:2023/6/9Data Set:% ([Book1]FitPeaks1,@WL,Input.IDTR1.IDTC2) Fitting Results Max Height 3.69041 4.61132 3.75572 3.08802 2.27507 1.41047 5.07069 Area IntgP 14.52062 19.50261 15.88403 13.06014 9.62191 5.9653 21.44538 FWHM 13.24856 13.24856 13.24856 13.24856 13.24856 13.24856 13.24856 Center Grvty 13.10505 48.10279 62.6084 76.49532 91.203 107.55738 34.50623 Area Intg 48.41935 65.03192 52.96568 43.54935 32.08447 19.89144 71.51012 Peak Type Gaussian Gaussian Gaussian Gaussian Gaussian Gaussian Gaussian Peak Index 1. 2. 3. 4. 5. 6. 7. Fig. S-6-1. NH3-TPD of CeO2/CNT PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7388 0 50 100 150 0 1x10 0 2x10 0 3x10 0 N H 3 -T P D s ig m a l o f Z n O /C N T (a ,u .) Time (min) Peak Analysis BaseLine:Line Adj. R-Square=-- # of Data Points=% ([Book1]FitPeaks1,@WL,RegStats.C1.N) Degree of Freedom=% ([Book1]FitPeaks1,@WL,RegStats.C1.DOF)SS=-- Chi^2=-- Date:2023/6/14Data Set:% ([Book1]FitPeaks1,@WL,Input.IDTR1.IDTC2) Fitting Results Max Height 0.67212 0.33974 0.72245 0.78662 0.19402 0.20662 0.19993 0.28055 Area IntgP 19.31057 10.0417 21.35376 23.25032 5.73477 6.10722 5.90943 8.29223 FWHM 6.58158 6.58158 6.58158 6.58158 6.58158 6.58158 6.58158 6.58158 Center Grvty 5.3626 31.65677 37.99558 44.13493 66.50297 75.9307 84.40371 11.68765 Area Intg 4.57712 2.38015 5.06141 5.51094 1.35929 1.44757 1.40069 1.96548 Peak Type Gaussian Gaussian Gaussian Gaussian Gaussian Gaussian Gaussian Gaussian Peak Index 1. 2. 3. 4. 5. 6. 7. 8. Fig. S-6-2. NH3-TPD of ZnO/CNT PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7389 0 50 100 150 0 1x10 0 2x10 0 3x10 0 N H 3 -T P D S ig n a l o f Z rO 2 /C N T ( a .u .) Time (min) Peak Analysis BaseLine:Line Adj. R-Square=-- # of Data Points=% ([Book1]FitPeaks1,@WL,RegStats.C1.N) Degree of Freedom=% ([Book1]FitPeaks1,@WL,RegStats.C1.DOF)SS=-- Chi^2=-- Date:2023/6/14Data Set:% ([Book1]FitPeaks1,@WL,Input.IDTR1.IDTC2) Fitting Results Max Height 2.43961 1.24324 0.77051 0.70975 Area IntgP 46.19561 24.56096 15.22193 14.0215 FWHM 10.22009 10.22009 10.22009 10.22009 Center Grvty 7.54388 19.04602 30.17556 42.7624 Area Intg 25.43874 13.52509 8.38233 7.72128 Peak Type Gaussian Gaussian Gaussian Gaussian Peak Index 1. 2. 3. 4. Fig. S-6-3. NH3-TPD of ZrO2/CNT PEER-REVIEWED ARTICLE bioresources.com Fan et al. (2023). “Metal oxides & hydrogenolysis,” BioResources 18(4), 7367-7390. 7390 0 50 100 150 0 1x10 0 2x10 0 3x10 0 4x10 0 N H 3 -T P D S ig n a l o f N b 2 O 5 /C N T ( a .u .) Time (min) Peak Analysis BaseLine:Line Adj. R-Square=-- # of Data Points=% ([Book1]FitPeaks1,@WL,RegStats.C1.N) Degree of Freedom=% ([Book1]FitPeaks1,@WL,RegStats.C1.DOF)SS=-- Chi^2=-- Date:2023/6/14Data Set:% ([Book1]FitPeaks1,@WL,Input.IDTR1.IDTC2) Fitting Results Max Height 3.16354 1.31788 0.28327 Area IntgP 66.21381 27.80877 5.97742 FWHM 8.98767 8.98767 8.98767 Center Grvty 9.20467 19.74008 46.6069 Area Intg 30.02079 12.60826 2.71011 Peak Type Gaussian Gaussian Gaussian Peak Index 1. 2. 3. Fig. S-6-4. NH3-TPD of Nb2O5/CNT